Characterization of Hadfield Steels Subjected to Various Heat-Treating Processes by Nondestructive Eddy Current Method

Abstract

A nondestructive eddy current method has been used to characterize microstructural features of Hadfield steels subjected to various conditions of heat treatment. The heat treatment parameters including quenching media (furnace, air, and water) and tempering temperatures (in the range of 200–600°C) were varied to obtain different microstructures. X-ray diffraction method, scanning electron microscopic observations, and hardness testing were utilized to characterize the phases formed in microstructure and mechanical properties. Quantitative values of carbide and pearlite structures were determined using an image processing software applied on microscopic images. Effect of microstructural features (type, fraction, and morphology of the formed phases) on the eddy current outputs (RMS voltage, normalized impedance, and phase angle) have been evaluated. The study demonstrates that the eddy current method could be used to detect microstructural changes of heat treated Hadfield steel, nondestructively. The results also show correlation coefficients of over 93% in determining austenitic matrix fraction by eddy current technique.